1. Field of the Invention
This invention relates generally to the field of packaging, and more specifically to the field of retortable plastic containers. More specifically, the invention relates to an improved retortable container that is more dimensionally stable during the sterilization process than conventional predecessor containers.
2. Description of the Related Technology
Certain products require sterilization during the packaging process in order to inhibit the growth of bacteria. Products requiring sterilization include foods such as milk, yogurt and various sauces, as well as certain pharmaceutical products. Thermal processing, sterilization, canning and retorting are all terms referring to the process of taking a food product, already sealed in its container, and heating it to a specific temperature for a specific time. The objective is to kill spoilage organisms and pathogenic bacteria, thus preserving the food and allowing it to be stored unrefrigerated for extended lengths of time.
There are multiple designs for retorting food containers, including batch systems and continuous systems. In a batch system, containers are placed in crates or baskets, which are then loaded into a vessel into which the heating medium is introduced. This method is the oldest and most traditional and also the most versatile in the range of products and container sizes it can handle. In a continuous retort system, a conveyor is used to continuously transport the containers to be sterilized through a heating chamber that contains the heating medium. There are advantages to each method depending on individual processing operations and, just as important, the type of food being processed.
Traditionally, products that require heat sterilization have been packaged in glass containers, which are relatively stable at elevated temperatures and pressures. However, in recent years plastic retortable containers have come into use. Plastic containers tend to be less expensive than glass containers and safer in many respects because they will not shatter when dropped. Unfortunately, plastic containers may lack the column strength that is necessary to avoid deformation of the sidewall of the container when a number of containers or palettes of containers are stacked during transportation or in packaging or retail facilities. While it is possible to increase the strength of a plastic container by increasing the thickness of the sidewall, doing so also increases manufacturing costs by increasing the amount of plastic material that is required. Lightweighting is an important consideration in the design of plastic containers, including plastic cans, because plastic material tends to be relatively expensive.
Many plastic containers also lack the requisite circumferential or hoop strength that is required to avoid excessive deformation when the contents of the container becomes pressurized, such as during a heat sterilization process.
The most common commercial procedure for heat sterilizing canned foods is a retort process in which filled but unsterilized sealed cans are placed in a retort chamber that is injected with steam and held at a predetermined elevated temperature (typically between about 210° F. to about 260° F.) for a predetermined period of time. Conventional plastic containers have been considered unsuitable for packaging applications in which heat sterilization is required, because the heat and pressurization that is inherent to such processes has the tendency to cause irreversible damage and deformation to the sidewall of the plastic can.
The temperatures of the retort process are elevated enough to temporarily increase the internal pressurization of the container. Plastic retortable containers accordingly have been designed to permit limited and reversible controlled flexure of one or more surfaces in order to accommodate the internal volumetric changes that are inherent to the retort sterilization process. U.S. Pat. No. 5,217,737 to Gygax et al. discloses a retortable plastic container that has a flexible bottom portion to accommodate internal volumetric changes. Other retortable containers that have been in commercial use have a champagne style bottom portion that is designed to permit a certain amount of flexure. However, when using a continuous retort process the flexure of retortable plastic containers must be limited so that it will not interfere with the process of conveying the container through the continuous retort system. Typically, such conveyors require at least two dimensionally stable points of contact on the container.
A need accordingly exists for an improved retortable container that exhibits improved dimensional stability and strength during the retort process without significantly adding to material costs.